Hi Everyone! I’m trying to figure out an rpm problem with my 2006 3.9l Monte. For a while, I seemed to notice a shaking when I was only at red lights. For a while, I was saying it might be a bad torque converter. I think I might be getting closer to the problem, but need some help. What I’ve been watching for the past few weeks have been the rpms at red lights, they’re always steady, but I finally noticed something different. Sometimes they were down around 600, which is when the shaking was the worst. Then on other drives, the red light rpms would be around 750, maybe 800, and the shaking seems to be gone. They stay pretty much steady each time, just either 600 with shaking, or 800 with no shaking. Does anyone have any ideas on what can cause a big change like this? Some drives start good, then get bad. I previously had a cam wipe out (which has been repaired), and am starting to think this might have been what was behind that. It seems to be very inconsistent, some days good, some days bad, so I’m not sure with where to begin. There have been no codes. I was thinking maybe something with the throttle body? A friend said maybe motor mounts (the dog bone inserts on mine seem a little worn, working on polyurethaning them). Any ideas on how to keep them the same on each stop would be great! Thanks!

 

You may have a misfire, a vacuum leak, or a clogged/faulty idle air control valve.

It could be a couple other things, too, but those are the most common and first things you should explore.

This is because:

If you have a misfire, one of your cylinders is not working and while that may be less noticeable at higher speeds when the engine is spinning really fast and the throttle is wide open, at lower speeds the difference is more apparent.

If there is no misfire (and no codes / check engine light), the problem is most likely air related. This could mean a vacuum leak from a hose or the intake manifold itself. Or, commonly, it could be that your Idle Air Control Valve is clogged or not working well.

The engine monitors via sensors how much air is coming in vs engine speed and uses that to control fuel injection and ignition timing.

If it's a vacuum leak, the engine is getting more air than it thinks it is and that is causing a 'lean' fuel/air ratio.

If the IAC valve (and its passages around the throttle plate) are clogged up, the engine thinks it's getting more air than it really is at idle with the throttle plate closed (foot off the gas), resulting in a 'rich' fuel/air ratio.

All of these scenarios will probably cause some sort of engine code and checking them should be your first step. Note that an oxygen sensor code will be indicitave of one of the above 'air' situations because it detects how rich or lean the ratio is.

 

I also suggest cleaning your throttle body and maf but it is more than likely a vacuum leak

 

Thanks for the info! I’ll slowly be going through each one of those making sure I didn’t miss anything. I could be wrong, but I don’t think this model has an IAC (that was actually my first guess when I first started noticing the problem). About a year ago I tried to clean it (I had a service record saying it was previously cleaned), but it doesn’t show up in any parts lists, and when I had the engine apart, it didn’t look like it was there. I’ll check for misfires again to be sure. In the meantime I’ll look more into the vacuum leaks, and post when I have the readings, or if I’m lucky enough to figure it out before!
The part I keep finding strange is sometimes it’s perfect, other times it’s not even close. Now that I’m going through this again, maybe TPS? I’ll look into the vacuum issue in the meantime, thanks for the suggestion!

 

Actually, testing the vacuum was much easier than I thought it would be. I got my reading using the Torque app after the car was up to operating temperature. When it was in park, it was consistently near 19.5 in/Hg, going down to 19.2 but constantly in that range. When I switched to drive and kept my foot on the brake, it seemed to stay around 17.5, going down to 17.2 (the RPMs were around 600, so there was shaking). I checked the specs, and it seems to say it should be between 17-22 when in park, so I guess that means this should be ok? I haven't done anything with identifying vacuum leaks before, so this is a little new to me. I'm going to check the misfires later or tomorrow.

 

Not sure what engine your car has but they sell and IAC for a 3.9 on rock auto.com here is a link

More Information for DORMAN 645405

and here is the link for a 3.5

More Information for DORMAN 645405

and for the 5.3

More Information for DORMAN 645405

so that could be the culprit.

It never hurts to clean the TPS as well

 

Thanks for the links! 3.9L. There is that adapter, but it goes with the TPS which is clipped on (I'll try to take pictures). I finally had a chance to clean the throttle body though, it still has the shaking The plate and around it are definitely clean now, it was a little dirty before. What I did notice, when the AC and heat blower fan is on, that is when the RPMs stay at 750, and there is no shaking. When I shut that off, they stay at 600. That seems to make all the difference with the shaking, so for now, I'm leaving the blower on until this is resolved. My code reader is also showing no misfires, so if it is misfiring, it isn't enough to register. I wish I would've noticed the RPMs and shaking related to the fan sooner.